Possible interactive risk of copper hydroxide, inorganic bactericide of fire blight with dinotefuran and triflumizole on honey bee

In spring 2021, abnormal honey bee mortality was observed near apple orchards in Cheongsong, South Korea, following copper-based fire blight control. This study evaluated the acute toxicity of copper hydroxide and basic copper sulfate to honey bees, assessed residue levels in field-collected bees, and investigated potential synergistic effects with co-occurring pesticides. Five-step serial dilutions of the recommended field concentration (RFC) were used for acute oral and contact toxicity assays. ICP-OES was used to quantify copper residues in lab-exposed and field-collected bees, while multi-residue pesticide analysis was performed with LC-MS/MS and GC–MS/MS. Copper hydroxide showed the highest oral toxicity, with both LD₅₀ and HQ values indicating moderate toxicity. Field-collected bees exhibited higher copper bioaccumulation than control, comparable to lab-exposed bees with a 10⁻² RFC dilution, which remained well under the LC₅₀. This implies that copper alone is not the cause of the witnessed phenomenon. Among detected pesticides, dinotefuran exhibited high HQ values, exceeding elevated risk threshold. Triflumizole and buprofezin showed minimal risk; however, triflumizole was considered likely to interact with dinotefuran, with their short half-lives potentially contributing to increased toxicity in bees. The combination of copper, dinotefuran, and triflumizole suggests potential interactive toxicity contributing to the observed mortality. These findings underscore the need to account for pesticide mixtures in practical management strategies, as even low-risk compounds can contribute to elevated toxicity when applied together. To prevent unintended harm to honey bees, a more integrated and pollinator-conscious approach to pesticide use is essential.